Presumably, something collided with Phoebe which caused its rather angled orbit and it is Phoebe's gravitational drag which has resulted in the ring. Iapetus (size ~1500 km) and Phoebe (size ~220 km) are the largest of the outer moons but Phoebe is nearly 4 times further out.

Regards, Rob.

glenc

08-10-2009, 02:48 PM

S&T has a diagram of it: http://media.skyandtelescope.com/images/Phoebe+ring+art.jpg
and a write up: http://www.skyandtelescope.com/news/63685982.html
also from NASA: http://www.jpl.nasa.gov/news/news.cfm?release=2009-150

Robh

08-10-2009, 03:02 PM

Thanks Glen,
So, it looks like the wanderer Phoebe was captured by Saturn, due to its retrograde angled orbit. The dust is due to one or perhaps multiple smaller impacts with the moon.